It does not defy human explanation. It's called the mothering instinct, and it's very powerful. Why is it that when a little blonde kid falls into a gorilla pit and is rescued by a female gorilla, we call the ape "almost human," and when a dog looks after a dumped baby girl it's called a "miracle," when really, those animals are just acting on the same instincts that should have kept that baby's parents from abandoning her in the first place?